Moondust in the Wind

April
10, 2008: Moondust is dry, desiccated stuff, and
may seem like a dull topic to write about. Indeed, you could
search a ton of moondust without finding a single molecule
of water, so it could make for a pretty "dry" story.
But like the dust in your mother's attic, moondust covers
something interesting – the moon – and even the dust itself
has curious tales to tell.

A
group of NASA and University of Alabama researchers are what
you might call "active listeners": Mian Abbas, James
Spann, Richard Hoover and Dragana Tankosic have been shooting
moondust with electrons, levitating moondust using electric
fields, and scrutinizing moondust under an electron microscope.
All this is happening at the National Space Science and Technology
Center's "Dusty Plasma Lab" in Huntsville, Alabama.

Why
such attention? Spann explains: "Humans will return to
the moon in a few years and have to know what to expect. How
do you live and work in a place filled with moondust? We're
trying to find out."

"Moondust
was a real nuisance for Apollo astronauts," adds Abbas.
"It stuck to everything – spacesuits, equipment, instruments."
The sharp-edged grains scratched faceplates, clogged joints,
blackened surfaces and made dials all but unreadable. "The
troublesome clinginess had a lot to do with moondust's electrostatic
charge."

Dust
on the moon is electrified, at least in part, by exposure
to the solar wind. Earth is protected from the solar wind
by our planet's magnetic field, but the moon has no global
magnetic field to ward off charged particles from the sun.
Free electrons in the solar wind interact with grains of moondust
and, in effect, "charge them up."

At
the Dusty Plasma Lab, the scientists simulate solar wind-like
conditions to study the moon's dust in a realistic environment.
In
previous
studies, Abbas and colleagues examined the effects of
ultraviolet sunlight on grains of moondust to help construct
theories about how moondust will behave during daylight hours
on the moon. (UV photons can also charge up moondust.) Now
they are investigating how the grains behave in the dark of
night, when the swirling solar wind dominates "lunar
weather."

"Fortunately,
we know what the solar wind is like, so we can simulate it,"
says Spann.

In
a typical experiment, Abbas peppers the dust grains with a
beam of electrons from an electron gun. He suspends a single
grain of moondust inside the vacuum test chamber and bombards
the grain with different numbers of electrons.

"We've
had some surprising results," says Abbas "We're
finding that individual dust grains do not act the same as
larger amounts of moon dust put together. Existing theories
based on calculations of the charge of a large amount of moondust
don't apply to the moondust at the single particle level."

Below:
Illuminated by red laser light, a single speck of moondust
hangs suspended in a vacuum chamber at the NSSTC's Dusty Plasma
Lab.

When
it comes to electrostatic charging, grains of moondust are
individualists capable of eccentric and surprising behavior.
For instance, in one experiment conducted by Abbas, pelting
a positively charged grain of moondust with electrons (which
carry a negative charge) caused the grain to exhibit a more
positive charge. Consider that grain a contrarian! Abbas thinks
that each electron hitting the grain dislodged two or more
electrons already there, resulting in a net increase of positive
charge.

Not
all moondust behaves this way. How each grain reacts depends
on a variety of factors including the grain's size, the charge
it already carries, and the number of free electrons incoming.

Spann
adds, "We believe the single grains will behave differently
on the moon, too – not just in our lab. Our results are closer
to what's really happening on the moon. We're saying, 'Hey
wait a second guys. We're finding something odd. When you
go to the moon, it's going to be a little different than you
thought.'"

You
can bet mission planners will be listening as the moondust
tells its tale.